Many kinds of voltage regulators have multiple amplifier stages and an output stage. The input amplifier stage provides an amplified error signal corresponding to the difference between a reference voltage input and a regulated voltage provided by the output stage. The error signal, after one or more subsequent stages of amplification, is applied to the regulator output stage. The amplifier error signal causes the regulator output stage to maintain a regulated voltage level regardless of changing load conditions. One or more amplifier stages included after the input stage provide error signal gain and isolate the regulator output stage from the input amplifier stage. Multiple amplifier stages are typically needed to provide sufficient gain before the error signal is applied to the output stage since the magnitude of the error signal controls current flow in the output stage. Otherwise, poor load regulation results for applications having high load conditions.
Fast load regulation is an important specification for a regulator. A regulator having a source follower driver provides fast load regulation. However, this type of regulator requires high voltage headroom to operate the source follower in saturation. The source follower driver is typically powered by a boosted supply voltage for low-voltage applications, which can be problematic. Replacing the source follower driver with a common-source driver overcomes the voltage headroom limitation. However, regulation speed is limited by the bandwidth of the amplifier feedback loop when a common-source driver is used.
The regulated voltage provided by an output stage of a multi-stage regulator may drive a capacitive load which can be high for many applications. When a high capacitive load is driven, the dominant pole of a regulator with a common source driver is set by the capacitive load. Each amplifier stage included in the regulator sets a non-dominant pole. A non-dominant pole close to the dominant pole affects the bandwidth (frequency range) of the multi-stage amplifier. Correspondingly, this non-dominant pole also affects the transient response time of the multi-stage amplifier. Voltage regulator performance suffers when the amplifier response time is not sufficiently fast, i.e., when the amplifier bandwidth is too low.
Amplifier bandwidth may be increased by decreasing the output resistance of the last amplifier stage which feeds the common-source output stage. Conventionally, this has been achieved by increasing the bias current and/or by increasing the device size of the stage. However, increasing bias current increases power consumption which may create thermal dissipation concerns. Increasing the device size of the last amplifier stage worsens parasitic capacitance, thus reducing amplifier stability.